JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-alpha/css/bootstrap.min.css">
	<div class="container-fluid">

		<div class="md-col-12">
		header
		</div>

    <div class="col-md-12">
    <div class="left-bar">
   <p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf </p><p>alfkj aslkdjf slkdjf lksajf lkasjd flkajsd flkja sdlfkj asldkfj laskdj flkasjdf...

CSS

html,
body {
  height: 100%;
  width: 100%;
  overflow-y: hidden;
  border:1px dashed gray;
}

div {
    height:100%; 
    border:1px solid green;
}


.left-bar{
	 // why I cannot use % for my height?
	width:40%;
  height:50%;
	border:1px solid green;

}